In general, an exhaust passage of an internal combustion engine is provided with an exhaust purification catalyst for purifying exhaust gas which is exhausted from the internal combustion engine. As such an exhaust purification catalyst, for example, an exhaust purification catalyst which has an oxygen storage ability is used. An exhaust purification catalyst which has an oxygen storage ability can remove the unburned gas (HC or CO etc.) or NOX etc. in the exhaust gas which flows into the exhaust purification catalyst when the oxygen storage amount is a suitable amount which is smaller than a maximum storable oxygen amount (maximum amount of oxygen which can be stored by exhaust purification catalyst). That is, if exhaust gas of an air-fuel ratio which is richer than the stoichiometric air-fuel ratio (below, also referred to as a “rich air-fuel ratio”) flows into the exhaust purification catalyst, the oxygen which is stored in the exhaust purification catalyst enables the unburned gas in the exhaust gas to be removed by oxidation. On the other hand, if exhaust gas of an air-fuel ratio which is leaner than the stoichiometric air-fuel ratio (below, also referred to as a “lean air-fuel ratio”) flows into the exhaust purification catalyst, the oxygen in the exhaust gas is stored in the exhaust purification catalyst. Due to this, the surface of the exhaust purification catalyst becomes an oxygen-deficient state. Along with this, the NOX in the exhaust gas is removed by reduction. As a result, the exhaust purification catalyst can purify the exhaust gas regardless of the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst so long as the oxygen storage amount is a suitable amount.
In this regard, the exhaust purification catalyst deteriorates as the length of use becomes longer. It is known that if the exhaust purification catalyst deteriorates in this way, along with this, the exhaust purification catalyst deteriorates in the maximum storable oxygen amount. For this reason, by detecting the maximum storable oxygen amount of the exhaust purification catalyst, it is possible to detect the degree of deterioration of the exhaust purification catalyst. As such a method of detection of the maximum storable oxygen amount, for example, it is known to perform active air-fuel ratio control which periodically switches the target air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst between the rich air-fuel ratio and the lean air-fuel ratio. In this method, the output during performance of the active air-fuel ratio control of the oxygen sensor which is provided at the downstream side in the exhaust flow direction of the exhaust purification catalyst is used as the basis to diagnose deterioration of the exhaust purification catalyst.
For example, in the abnormality diagnosis system which is described in PLT 1, in the period when the target air-fuel ratio is set to the rich air-fuel ratio or lean air-fuel ratio, the period of the active air-fuel ratio control is set so that the cumulative value of the amount of unburned gas or the amount of oxygen in the exhaust gas which flows into the exhaust purification catalyst becomes an amount between the maximum storable oxygen amount (breakthrough amount) when the exhaust purification catalyst is normal (when the degree of deterioration is small) and the maximum storable oxygen amount when the degree of deterioration of the exhaust purification catalyst is large. Further, when the output of the downstream side oxygen sensor greatly swings, it is judged that the exhaust purification catalyst has deteriorated, while when the swing is small, it is judged that the exhaust purification catalyst has not deteriorated.